Corona discharge reactor

ABSTRACT

A modular corona discharge reactor in which individual reactor chambers are arranged vertically and are grouped in serial pairs.

The present invention relates to reactors for the processing ofmaterials in or carried by a gaseous phase by means of coronadischarges.

BACKGROUND OF THE INVENTION

Considerable effort is being expended on the development of techniquesfor carrying out processes in the gaseous phase, using species whichhave been activated by corona discharges. Corona discharges occur ingaseous media when the localised electric field in the neighbourhood ofa body exceeds the electrical breakdown voltage of the gaseous medium.Some existing corona discharge reactors consist of a chamber having aninlet and an outlet for a gaseous medium, an axial inner electrode and acylindrical outer electrode surrounding the inner electrode. Theelectrical discharge within such reactors consists of streamersextending from the inner electrode towards the outer electrode. For theeffective processing of a gaseous medium passing through the reactor, itis necessary to produce as many corona streamers as possible because anyspace which does not contain such streamers is dead space as far as theprocessing of the gaseous medium is concerned.

In one type of known corona discharge reactor the central electrode isin the form of a wire. However, the rapid fall-off in the electric fieldin a central wire electrode type of corona discharge reactor means thatthe distribution of streamers is very sensitive to wire position. Thisphenomenon limits the efficiency with which gaseous medium passingthrough the reactor can be treated.

Existing central wire electrode corona discharge reactors have diametersof a few centimetres and lengths of a few tens of centimeters. Toprocess reasonable volumes of gas, high flow rates are required, whichin turn tend to increase the mechanical instability of the centralelectrode due to aerodynamic effects as well as leading to shortresidence times in the reactor chamber of the medium to be processed.Hence, existing corona discharge reactors are limited inherently intheir effectiveness.

Other corona discharge reactors, see for example our earlier patent GB 2282 738, U.S. Pat. No. 5,041,145, U.S. Pat. No. 5,268,151 or U.S. Pat.No. 4,966,666, make use of central electrodes which have a largerdiameter. Among other things this reduces the electric field gradient inthe region of the central electrode at the same voltage, but there stillremains a limitation on the separation between the inner and outerelectrodes if an effective corona discharge is to be maintained. Merelyincreasing the length of corona discharge reactors does not provide ananswer to the problem because the gas flow resistance becomes excessive.Also, corona discharge reactors operate in a pulsed manner, and the timetaken for an energising pulse to traverse the length of the centralelectrode, provides another factor which limits the length of a coronadischarge reactor.

GB specification 2 008 369A discloses an ozone generator which includesa,plurality of parallel electric discharge chambers each of which has acentral wire electrode. The wires are supported at their ends by twogrid structures to one of which a common feed wire is connected towhich, in use a d.c. voltage is applied.

As the device is operated in a d.c. mode, no a.c. current distributioneffects have to be considered.

GB patent 1 589 394 discloses an ozone generator which includes a numberof parallel corona discharge chambers, a single power supply, which mayproduce pulsed d.c., a.c., or a mixture of both, potentials is used, butno attempt is made to equalise the distribution of the power supplied tothe discharge chambers, either in terms of magnitude or time.

U.S. Pat. No. 4,495,043 discloses an ozoniser in which there is aplurality of ozone producing chambers which are connected to a singlepulsed a.c. power supply. However, the ozone producing chambers are notoperated simultaneously or, continuously, but are operated sequentiallyin a pulsed a.c. mode, the duration of each pulse being related to thepassage time of a pulse of ozone-producing gas through the correspondingozone producing chamber, and the intervals between the pulses: appliedto a given ozone-producing chamber being such that ozone produced by onepower pulse is cleared from the chamber before the next power pulse isapplied to that ozone producing chamber.

The patent is concerned mainly with the design of the power supply. Thequestion of the electrical relationship between the ozone-producingchambers is not addressed at all.

U.S. Pat. No. 5,009,858 discloses an ozoniser in which ozone is producedby a silent electric discharge in a number of chambers operated inparallel from a common power supply. The electrical relationship betweenone ozone producing chamber and another is not discussed at all, but itwould appear that the ozone producing chambers are operated in acontinuous a.c. mode.

WO 99/15267 discloses a corona discharge reactor for processing agaseous medium. The gaseous medium is fed via inlet and outlet plenumchambers through an array of reactor chambers. Each reactor chamber hasan inner axial electrode and is formed as a longitudinal, circularsection, gas passage through an electrode assembly, which thus provideseach reactor chamber with an outer electrode concentric with its innerelectrode. Pulsed electrical power is applied simultaneously orsequentially to the inner electrodes of each of the reactor chambers.There is, however, no disclosure relating to balancing the flow ofgaseous medium through the reactor chambers.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved form ofcorona discharge reactor for use in the processing of gaseous media bymeans of electrically activated species.

The term gaseous medium includes the case when one or more constituentsof the medium is in the form of an aerosol, or finely divided solidmatter carried by a gaseous phase.

According to the present invention there is provided a modular coronadischarge reactor for the processing of gaseous media, comprising aninlet duct and an outlet duct, a plurality of vertically mounted coronadischarge reactor chambers positioned between the inlet and outlet ductsand communicating therewith, each corona discharge reactor having acylindrical outer electrode structure and an axial inner electrodestructure, a high voltage pulse power supply positioned in close coupledvertical alignment with the inner electrode of the reactor chamber andconnected directly thereto, each high voltage pulse power supply beingadapted to produce voltage pulses of a magnitude sufficient to excite acorona discharge in a gaseous medium to be processed as it passesthrough the associated reactor chamber, means for balancing the flowthrough the reactor chambers of the gaseous medium to be processed, alow voltage power supply connected to each high voltage pulse powersupply and a control unit adapted to control the action of the lowvoltage power supply to cause the high voltage pulse power supplies toapply high voltage pulses to the corona discharge reactor chambers witha pre-determined temporal relationship.

In a particular embodiment of the invention, the reactors are arrangedinto groups of two reactors in series between the inlet and outlet ductsand the means for balancing the flow through the reactor chambers of thegaseous medium to be processed comprises a flow control device situatedbetween the reactor chambers of each pair of reactor chambers.

A simple form of flow control device is a calibrated orifice plate.

As corona discharge reactors are noisy devices, electromagnetically,preferably at least the corona discharge reactor chambers are providedwith electromagnetic screening.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of an embodiment of theinvention,

FIG. 2 is a front elevation of the embodiment of the invention shown inFIG. 1, and

FIG. 3 is a longitudinal section through a corona discharge reactorchamber, included in the embodiment of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2 of the drawings, a modular corona dischargereactor 1 consists of an inlet duct 2 and an outlet duct 3. Positionedbetween the inlet duct 2 and the outlet duct 3 are ten corona dischargereactor chambers 4 (described below) arranged in five groups of pairs ofreactor chambers 4 in series between the inlet duct 2 and the outletduct 3. Each reactor chamber 4 in a pair is joined by a length 5 ofducting which terminates in a coupling flange 6. Sandwiched between thecoupling flanges 6 is an orifice late 7, which also acts as a gasketbetween the coupling flanges 6. The holes in the orifice plates 7 arearranged on initial assembly of the reactor 1 to equalise the flowthrough the reactor chambers 4 of a gaseous medium to be processed inthe reactor 1. If so desired, the orifice plates 7 can be replaced byadjustable valves, such as gate valves, so giving a capability formonitoring the gas flow rates through each pair of reactor chambers 4and/or the processing efficiency of the reactor chambers 4 and varyingthe flow rates through the reactor chambers 4 to optimise the processingof the gaseous medium by the reactor 1 as a whole. Mounted beneath eachreactor chamber 4 is a high voltage pulse power supply 8 which isconnected directly to an extension 9 of a central electrode of eachreactor chamber 4. The connection extensions 9 between the reactorchambers 4 and the high voltage pulse power supplies 8 pass through theinlet and outlet ducts 2 and 3 respectively, in insulating sleeves whichare held in position in the ducts 2 and 3 by sealing collars 10. Thereactor chambers 4 and the high voltage pulse power supplies 8 areenclosed in metal screens 11 to minimise the emission of electromagneticnoise from the reactor 1 when it is operating. In the embodiment of theinvention described, the metal screens 11 are diamond-formed sheets ofmetal which are mounted on a frame 12 and sealed.

The high voltage pulse power supplies 8 are connected to a control unit13 which is arranged to apply activating pulses from a low voltage powersupply to the high voltage pulse power supplies 8 to cause them to applyhigh voltage pulses via connection extensions 9 to the centralelectrodes of the reactor chambers 4 with a pre-determined temporalrelationship (usually simultaneously).

Referring to FIG. 3 of the drawings, each reactor chamber 4 consists ofan outer cylindrical casing 301 within which there is an axial centralelectrode assembly 302 and a co-axial outer electrode assembly 303. Thecasing 301 of the reactor chamber 4 has top and bottom flanges 304, 305,respectively. The top flange 304 is used to connect the reactor chamber4 in conjunction with a flange 306 to the ducting 5 linking each pair ofreactor chambers 4. The bottom flange 305 is used to connect the reactorchamber 4 to a flange 307 by which the reactor chamber 4 can be mountedupon whichever of the ducts 2 or 3 is appropriate. Positioned betweenthe flanges 304, 306 and 305, 307 are upper and lower central electrodesupports 308, 309, respectively.

The lower central electrode support 309 also serves to locate the outerelectrode assembly 303. The upper and lower electrode supports 308, 309have flown straightening holes 310 formed in them, and are made of amaterial which is both insulating and sufficiently deformable also toact as gaskets between the appropriate pairs of flanges 304, 306 and305, 307. A suitable material for use as the electrode supports 308, 309at gas temperatures of up to approximately 300° C. ispolytetrafluoroethylene.

The inner electrode assembly 302 consists of a tube 311 made of thealloy known as HASTELLOY C upon which there is mounted a stack of disks312 separated by lengths of tubing 313, both of which also are made ofHASTELLOY C. The disks 312 have a diameter larger than that of theseparator tubes 313. The lowest disk 312 rests upon a collar 314 formedat the upper end of the insulating sleeve 315 which surrounds theextension 9 of the inner electrode assembly 302 as it passes through theduct 2 or 3. The upper end of the inner electrode assembly 302 islocated in the upper electrode support 308 by means of a locating spigot316, which also is made of polytetrafluoroethylene.

The outer electrode assembly 303 is made of a stack of disks 317separated by tubes 318 which are mounted on, and clamped together bytwelve regularly spaced rods 319 and nuts 320. The whole outer electrodeassembly 301 also is made of the alloy HASTELLOY C.

The vertical arrangement of the reactor chambers 4 and the directlyconnected high voltage pulse power supplies 8 is not only efficientelectrically, but also ensures that no bending stresses are imposed uponthe reactor chambers 4, and in particular, upon the central electrodes302 of the reactor chambers 4, so ensuring that the gaps between thedisks 312 and 317 of the inner and outer electrode assemblies 302, 303,respectively do not alter.

The in-series pairs of reactor chambers 4 decreases the floor areaoccupied by the reactor and also enables the inlet and outlet ducts 2and 3 to be placed close together, which also is operationallyconvenient.

The invention is not restricted to the details of the foregoing example.For instance, an arrangement of ten corona discharge reactor chambers 4is described. It will be appreciated that more or less than ten may beemployed according to the operating performance required. The use ofHASTELLOY C for the inner electrode 302 is described, but other metalsor metal alloys may be employed depending upon the gaseous environmentin which the electrode is required to operate.

What is claimed is:
 1. A modular corona discharge reactor for theprocessing of gaseous media, comprising an inlet duct and an outletduct, characterised by the features in combination comprising aplurality of vertically mounted corona discharge reactor chamberspositioned between the inlet and outlet ducts and communicatingtherewith, each corona discharge reactor having a cylindrical outerelectrode structure and an axial inner electrode structure, a highvoltage pulse power supply positioned in vertical alignment with theinner electrode of the reactor chamber and connected directly thereto,each high voltage pulse power supply being adapted to produce voltagepulses of a magnitude sufficient to excite a corona discharge in agaseous medium to be processed as it passes through the associatedreactor chamber, means for balancing the flow through the reactorchambers of the gaseous medium to be processed, a low voltage powersupply connected to each high voltage pulse power supply and a controlunit for controlling the action of the low voltage power supply to causethe high voltage pulse power supply to apply high voltage pulses to thecorona discharge reactor chambers with a pre-determined temporalrelationship.
 2. A corona discharge reactor according to claim 1,wherein the reactor chambers are arranged as a series of one or morepairs of reactor chambers and the means for balancing the flow throughthe reactor chambers of the gaseous medium comprises a flow controldevice situated between the reactor chambers of the or each pair ofreactor chambers.
 3. A corona discharge reactor according to claim 2,wherein the or each flow control device comprises a calibrated orificeplate.
 4. A corona discharge reactor according to claim 2, wherein theor each flow control device comprises an adjustable valve.
 5. A coronadischarge reactor according to claim 1, wherein at least the reactorchambers and their associated high voltage pulse power supplies arescreened electromagnetically.